ISIS mounts fierce defence of besieged Syrian bastion

Islamic State group jihadists have launched a wave of suicide and car bombings to defend a besieged stronghold in northern Syria against US-backed fighters, a monitor said on Saturday.

A Kurdish-Arab alliance last week encircled the city of Manbij and severed a key supply route used by IS from the Turkish border to the jihadists' de facto Syrian capital, Raqa.

But since then the Syrian Democratic Forces (SDF), supported by US air strikes, have been slowed by almost daily suicide bombings by IS, the Syrian Observatory for Human Rights said.

On Friday, IS carried out two suicide attacks and five car bombings in the southwestern suburbs of Manbij, according to the British-based monitor, which relies on a network of sources on the ground. The number of casualties was unknown.

The operation has also been complicated by the presence of tens of thousands of civilians trapped in the city, although more than 1,000 have managed to escape with the help of the SDF.

"On Friday, six members of the same family were killed when they were targeted by jihadists while fleeing," Observatory head Rami Abdel Rahman said.

SDF commanders have accused IS of using civilians as human shields.

Southeast of Manbij, regime forces backed by Russian air strikes have also faced IS counterattacks after advancing towards another jihadist bastion, Tabqa.

The town lies around 50 kilometres west of Raqa city, and recapturing it would cut a key supply route. The army is now reported to be 15 kilometres away from Tabqa's military airport.

The Syrian conflict has killed more than 280,000 people and displaced millions since it began in 2011 with the brutal repression of anti-government demonstrations.

It has evolved into a multi-sided civil war involving a range of combatants including Western- and Gulf-backed rebels, jihadists, Kurds and pro-regime forces supported by Russia and Iran.